In a communications network it is common for each network element to issue routing data packets which are then distributed to the other network elements in the network. Each routing data packet normally carries information about the network element that issued it. By gathering the information carried by received routing data packets, a network element can learn about the other network elements in the network.
The International Organization for Standardization (ISO) standard ISO 10589 specifies procedures for the transmission of configuration and routing information between network elements. The procedures are defined in terms of the interactions between network elements in the form of Intermediate System (IS) network entities through the exchange of protocol data units (PDUs), including routing data packets in the form of Link State PDUs (LSPs). ISO 10589 is hereby incorporated herein by way of reference.
ISO/IEC 10589 includes a specification of what is commonly known as Intermediate System-to-Intermediate System (IS—IS) routing protocol. Section 7.2.3 of ISO 10589 relates to IS—IS routing protocol in the context of networks which comprise a broadcast sub-network, for example an Ethernet LAN (Local Area Network). It is stipulated that if a plurality of IS network elements, serving as IS—IS routers (hereinafter “routers”), are connected on a broadcast sub-network, then one of the routers must be elected to serve as a Designated Router (DR). The DR produces a pseudonode LSP for representing all of the other routers on the broadcast sub-network. The routers on the sub-network advertise, in their respective LSPs, an adjacency with the pseudonode rather than with one or more other actual routers.
In the event that a DR relinquishes its status as a DR, it is required to initiate a network-wide purge of its pseudonode LSP. In addition, a replacement DR must be elected, its pseudonode LSPs must be propagated through the network and each router in the network must assimilate resulting new routing information. During this time, some routers in the network will be unable to communicate with other routers in the network. It is desirable therefore to minimize the amount of LSP purging that is required in order to minimize the disruption to the network.
Recent developments in telecommunications technology allow routers which support a first set of communications protocols to exist on a network, or sub-network, with other routers which do not support the first protocol set. For example, a router which supports only OSI (Open system Interconnection) protocols may be connected to a LAN which also includes a router which supports only IP (Internet Protocol) protocols. Such developments are described in ITU-T (International Telecommunications Union Telecommunications Standardization Sector) recommendation G.7712.
In such cases, it is required to elect a DR in respect of each protocol set supported by routers on the network. This can complicate the purging of pseudonode LSPs and increase the disruption caused to the network. It is desirable, therefore, to provide an efficient way of purging pseudonode LSPs in a mixed protocol network.
It will be understood that the term “protocol” as used herein is intended to embrace protocol set, or protocol stack, where the set (or stack) may comprise one or more protocols. For example, OSI and IP may each be considered to comprise a respective set of protocols, but may be referred to herein as OSI protocol or IP protocol respectively.
The present invention is described hereinafter in the context of OSI and IP protocols, although it is not limited to use with such. Respective ISO (International Organisation for Standardisation) and IETF (Internet Engineering Task Force) standards describing these protocols are publicly available.
OSI traffic comprises OSI data packets, where OSI data packets conform with OSI protocol, particularly CLNP (ConnectionLess mode Network Protocol), CLNP being an OSI network layer protocol. CLNP is the name given to the type of data packets or PDUs (Protocol Data Units) that are used to provide CLNS (ConnectionLess mode Network Service). CLNS is the service provided by the network layer of an OSI protocol stack to higher layers of the stack. Provision of CLNS service results in CLNP packets or PDUs being passed to lower layers of the stack.
IP traffic comprises IP data packets, where IP data packets conform with IP protocol, particularly IPv4 and IPv6 which are IP network layer protocols.
An IP-only network element, or node, is a node that can natively route IP packets but not OSI packets. An OSI-only node is a node that can natively route OSI packets, but not IP packets. A dual, or bi-lingual, network element, or network node, is a node that can natively route at least two protocols, particularly network layer protocols. This term is used hereinafter particularly to indicate either a node that routes both CLNS/CLNP and IPv4, or alternatively a node that routes both IPv4 and IPv6.
It will be understood that the term “router” as used hereinafter is intended to embrace a network element, network entity or network node (or part thereof) that is arranged to act as a data router. Hence, a “dual router” is a network node (or part thereof) that is capable of routing at least two protocol sets.
An adjacent network node (“adjacency”) is a reachable neighbouring node. The term “adjacency” is defined in section 3.6.3 of ISO/IEC 10589 and may be used herein to denote a reachable neighbouring node. A physical neighbouring node is not necessarily a valid adjacency, since it might be in a different network area or network level. Thus, an adjacency may be maintained between nodes that are not physical neighbours.